Measurement of Temperature

1. Measurement of Temperature • Defining and measuring temperature • Thermal Time Constant • Measurement Errors • RTD’s • Thermistors • I.C. Sensors • Thermocouples Practical Temperature Measurement Temperature Measurement Presentation

2. Defining Temperature • A scalar quantity that determines the direction of heat flow between two bodies • A statistical measurement • A difficult measurement • A mostly empirical measurement http://www.m-w.com/dictionary.htmTemperature: degree of hotness or coldness measured on a definite scale http://www.m-w.com/dictionary.htm Empirical: originating in or based on observation or experience

3. Temperature Systems The Reaumur temperature scale is named after the French scientist (1683-1757). He proposed his temperature scale, in 1731. Reaumur divided the fundamental interval between the ice and steam points of water into 80 degrees, fixing the ice point at 0 Degrees and the steam point at 80 degrees. The reaumur scale, although of historical significance, is no longer in use.

4. Measuring Temperature • Don't let the measuring device change the temperature of what you're measuring. • Response time is a function of • Thermal mass (mass of the device e.g large Thermistor vs small Thermistor) • Measuring device (type of device e.g. RTD or Thermocouple) • The Thermal Time Constant for a thermistor is the time required for a thermistor to change its body temperature by 63.2% of a specific temperature span when the measurements are made under zero-power conditions in thermally stable environments.

5. Thermal Time Constant The dominant factors that affect the T.C. of a thermistor are: • The mass and the thermal mass of the thermistor itself. • Custom assemblies and thermal coupling agents that couple the  thermistor to the medium being monitored. • Mounting configurations such as a probe assembly or surface mounting. • Thermal conductivity of the materials used to assemble the thermistor  in probe housings. • The environment that the thermistor will be exposed to and  the heat transfer characteristics of that environment. • Typically, gases are less dense than liquids so thermistors have greater time constants when monitoring temperature in a gaseous medium than in a liquid one. http://www.betatherm.com/t_c.html

6. Thermal Time Constant 750C • Example: A thermistor is placed in an oil bath at 25°C and allowed to reach equilibrium temperature. The thermistor is then rapidly moved to an oil bath at 75°C. The T.C. is the time required for the thermistor to reach 56.6°C (63.2% of the temperature span [difference]). 56.60C 250C 3τ 1τ 2τ 4τ 5τ

7. Temperature Errors 95% Confidence interval • What is YOUR normal temperature? • Thermometer accuracy, resolution • Contact time • Thermal mass of thermometer, tongue • Human error in reading http://www.amstat.org/publications/jse/v4n2/datasets.shoemaker.html

8. The Resistance Temperature Detector (RTD) • RTD: Most accurate, Most stable, Fairly linear • Expensive (platinum) • Slow (relative) • Needs I source (changing resistance) • Self-heating (don’t change the measurement due to the internal current!) • 4-wire measurement (must take the resistance of the leads into account) http://www.temperatures.com/sensors.html http://www.minco.com/sensorsg.php

9. RTD’s http://www.efunda.com/designstandards/sensors/rtd/rtd_intro.cfm • RTDs are among the most precise temperature sensors commercially used. They are based on the positive temperature coefficient of electrical resistance. http://www.omega.com/ http://www.sensorsmag.com/articles/article_index/index.htm

10. RTD Linearity R=RRef[1+α(T-TRef)] R=100[1+.00385(70-60)] =103.85 ohms

11. RTD Measurement DDC RTD Measurement To balance the bridge: R1R3=R2R4 Dissipation ConstantThe power in milliwatts required to raise a thermistor 1°C above the surrounding temperature is the dissipation constant.

12. http://www.tiptemp.com/sense/Sense_RTD_TechData.pdf 4-wire circuit To estimate leadwire error for a 2-wire configuration, multiply the total length of the extension leads by the resistance per foot in the table shown below. Then divide by the sensitivity of the RTD, given in the table below to obtain an error in C°. Example: You are using a 100 platinum RTD with a TCR of 0.00385 and 50 ft. of 22 AWG leadwire. R = 50 ft. x 0.0165/ft. = 0.825 Approximate error = 0.825 / 0.385 = 2.14°C

13. Thermistors NTC Thermistor Shown RT R25 • Advantages: • High output • Fast • 2-wire measurement • Disadvantages • Very nonlinear • Limited range • Needs I source • Self-heating • Fragile http://www.embedded.com/story/OEG20020125S0100

14. Thermistors • Commonly used for sensing air and liquid temperatures in pipes and ducts, and as room temperature sensors.  Unlike RTD's, the temperature-resistance characteristic of a thermistor is non-linear, and cannot be characterized by a single coefficient.   • The following is a mathematical expression for thermistor resistance1: R(T) = R0 exp[b (1/T - 1/T0)] • Where: R(T) = the resistance at temperature T, in K, R0 = the resistance at reference temperature T0, in K, b = a constant that varies with thermistor composition T = a temperature, in K, T0 = a reference temperature (usually 298.15 K) • Because the lead resistance of most thermistors is very small in comparison to sensor resistance, three and four wire configurations have not evolved.  Otherwise, sensing circuits are very similar to RTD's, using the Wheatstone bridge 1Beckwith, Thomas G., Roy D. Marangoni, and John H. Lienhard V. Mechanical Measurements. New York: Addison and Wesley, 1993. Pp. 673 DDC Thermistors

15. ThermistorEquation http://www.omega.com/Temperature/pdf/44000_THERMIS_ELEMENTS.pdf

16. Thermistor Curvehttp://www.workaci.com/pdf/t-19.pdf

17. Eq. Temp 273.15 The Omega Thermistor equation is: 1/T =A+B*Ln(R)+C*(Ln(R))3 Thermistor Circuit .003661=A+8.903B+705.65C .0030945=A+6.698B+300.52C .0026799=A+5.0293B+127.21C To use this equation you write 3 simultaneous eqs. In 3 unknowns and solve. The eqs. Used the values at 0, 50 and 100 Celsius, with the Kelvin values shown on the left. 323.15 The final equation is: 1/T =A+B*Ln(R)+C*(Ln(R))3 with A = 1.472E-3, B=237.5E-6, and C=105.9E-9 The resulting temperatures from the equation are shown here and are almost identical to the given values. The resulting graph from the Eq. is indistinguishable From the original graph from the table. 373.15

18. I.C. Sensors • Advantages • High output • Most linear • Inexpensive • Disadvantages • Limited variety • Limited range • Needs V source • Self-heating

19. I.C. Sensors LM34: $2.33 from DigiKey AD590: $5.24 from Analog Devices • AD590 (Analog Devices) • Current Output – Two Terminal IC Temperature Transducer • Produces an output current proportional to absolute temperature. For supply voltages between +4 V and +30 V the device acts as a high impedance, constant current regulator passing 1 µA/K. • LM34 (National Semiconductor) • The LM34 is a precision integrated-circuit temperature sensor, whose output voltage is linearly proportional to the Fahrenheit temperature.

20. AD590 & LM34Circuits

21. Conversion from Kelvin to Fahrenheit We know that 273.150K = 00C = 320F AND 373.150K=1000C=2120F so we can write two linear equations in two unknowns. 32 = 273.15m + b 212=373.15m + b Solving these for m and b yields: m = 1.8 b = -459.67 the linear conversion equation is 0F = 1.8*(0K) – 459.67

22. AD590 Conversion to Fahrenheit in mV +10 -.45967volts 180KΩ 180KΩ - 100KΩ AD590 -1mV/0F 1mV/0K + 1KΩ 0F = -1.8*(0K) + 459.67 in mvolts Use an inverting amplifier to get positive output

23. Thermocouples • Advantages: • Wide variety • Cheap • Wide T. range • No self-heating • Disadvantages • Hard to measure • Relative T. only • Nonlinear • Special connectors

24. Seebeck and Peltier Effects

25. Seebeck coefficient   in a circuit exhibiting the Seebeck effect, the ratio of the open-circuit voltage to the temperature difference between the hot and cold junctions. Thermocouples

26. Thermocouples

27. Thermocouples

28. Thermocouples

29. Thermocouples

30. Summary • Defining and measuring temperature • Thermal Time Constant • Temperature Errors • RTD’s • Thermistors • I.C. Sensors • Thermocouples • Next